I. Field of the Invention
The present invention relates to multiple access communication systems, such as wireless data or telephone systems, and satellite repeater type spread spectrum communication systems. More particularly, the invention relates to a communication system architecture in which digital signal demodulation is performed using multiple digital receiver modules coupled to each of several analog receivers to decrease data transfer requirements. The invention further relates to a method of redistributing certain signal demodulation functions in a code division multiple access spread spectrum type communication system to decrease the rate of data transfer required to produce single user data signals.
II. Description of the Related Art
A variety of multiple access communication systems has been developed for transferring information among a large number of system users. The techniques employed by such multiple access communication systems include time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes, such as amplitude companded single sideband (ACSSB), the basics of which are well known in the art. However, spread spectrum modulation techniques, such as code division multiple access (CDMA) spread spectrum techniques, provide significant advantages over the other modulation schemes, especially when providing service for a large number of communication system users. The use of CDMA techniques in a multiple access communication system is disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", is assigned to the assignee of the present invention, and is incorporated herein by reference.
The 4,901,307 patent discloses a multiple access communication system technique in which a large number of generally mobile or remote system users each employs a transceiver to communicate with other system users or desired signal recipients, such as through a public telephone switching network. The transceivers communicate through satellite repeaters and gateways or terrestrial base stations (also sometimes referred to as cell-sites or cells) using code division multiple access (CDMA) spread spectrum type communication signals. Such systems allow the transfer of various types of data and voice communication signals between system users, and others connected to the communication system.
Communication systems using spread spectrum type signals and modulation techniques such as disclosed in U.S. Pat. No. 4,901,307, provide increased system user capacity over other techniques because of the manner in which the frequency spectrum is `reused` many times across different regions serviced by the system and concurrently among system users within a region. The use of CDMA results in a higher efficiency in utilizing a given frequency spectrum than achieved using other multiple access techniques. In addition, the use of wide band CDMA techniques permits such problems as multipath fading to be more readily overcome, especially for terrestrial repeaters.
Pseudonoise (PN) modulation techniques used in wide band CDMA communications provide a relatively high signal gain which allows spectrally similar communication channels or signals to be more quickly differentiated. This allows signals traversing different propagation paths to be readily distinguished, provided any path length difference causes relative propagation delays in excess of the PN chip duration, that is, the inverse of the bandwidth. If a PN chip rate of say approximately 1 MHz is used, the full spread spectrum processing gain, equal to the ratio of the spread bandwidth to system data rate, can be employed to discriminate between signal paths differing by more than one microsecond in path delay or time of arrival.
The ability to discriminate between multipath signals greatly reduces the severity of multipath fading, although it does not typically eliminate it due to occasional path delay differentials of less than a PN chip duration. The existence of low delay paths is more especially true for satellite repeaters or directed communication links because multipath reflections from buildings and other terrestrial surfaces are greatly reduced, and the overall path is so large. Therefore, it is desirable to provide some form of signal diversity as one approach to reduce the deleterious effects of fading and additional problems associated with relative user, or satellite repeater, movement.
Generally, three types of diversity are produced or used in spread spectrum type communication systems, and they are time, frequency, and space diversity. Time diversity is obtainable using repetition and time interleaving of signal components. A form of frequency diversity is inherently provided by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space diversity is provided using multiple signal paths, typically through different antennas or beams.
The beams used in satellite repeater communication systems are typically configured to cover larger geographic regions and, therefore, potentially address a larger number of users at any given time than terrestrial repeater systems. Each satellite generally employs multiple beams, on the order of eleven to sixteen, to reach several contiguous geographical regions at one time, and provide diversity. The relative size of the targeted subscriber audience in each beam is generally large even where the areal density of subscribers is small. That is, even though service areas might encompass land regions with low population densities, the overall size of each region means there is still a significantly large number of subscribers within a given satellite beam pattern. In addition, satellites are used in some geographical regions to overcome a lack of land based telephone systems, and such regions may have relatively high population densities.
Providing service to larger numbers of subscribers using satellites translates to both more effective transmitters or antennas per repeater, up to 16 beams per satellite, and more communication channels per satellite beam. Typically, the number of communication channels or circuits per beam in a satellite is increased to 128 channels from the 64 typically found in terrestrial repeaters. These factors greatly increase the amount of data and signal processing that must be accommodated within a system gateway as opposed to base stations.
Terrestrial base stations generally use no more than six antennas, ranging anywhere from one for an entire cell to two per each of three sectors in a subdivided cell, each receiving communication signals on one carrier frequency. Satellite gateways, on the other hand, handle communication signals using an array of receivers, on the order of 32 or more, with one or possibly more, as desired, antennas to accommodate sixteen or more beams or spots on different carrier frequencies. Gateways also provide service to multiple satellites which are `in view`, typically on the order of four at any given time. In one exemplary system, on the order of six satellites are used in each of eight orbital planes and even more satellites are contemplated for some systems.
The larger number of communication signals being accommodated in satellite type repeater systems translates into large amounts of data to be transferred through and processed within each gateway. When signals received by each antenna are downconverted to an appropriate baseband frequency and the carrier removed to provide digital samples, the data rates are on the order of 80 megabits-per-second (Mbps) per carrier frequency (beam). The signals from each analog receiver are transferred to an array of modems within the gateway which are assigned to process communications for particular subscribers. This means that data from each receiver must be transferred along common busses connected to all modems in order that they can each detect and process multi-path signals. For the current data rates within communication systems, the gateway busses transferring signals between analog receivers and modem sections of a gateway would have to handle on the order of several gigabits-per-second (Gbps) or more. The control, switching, timing, etc., for this much data is beyond the limits of cost effective gateway systems. This strains bus transfer structures beyond the limits of current technology within reasonable cost and reliability constraints. In addition, cabling requirements for transferring this volume of data among various processing circuit structures also becomes prohibitively complex.
Therefore, it is desirable to reduce the quantity of data that must be transferred from one functional element or stage to another within the architecture of a gateway. It is also desirable to make more efficient use of lower cost modular components that provide for ready expansion of systems, as capacity is increased or updating is needed.